JPH0935245A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium ensuring satisfactory electromagnetic transducing characteristics even when high density recording is performed. SOLUTION: A nonmagnetic layer as a lower layer 4 and a magnetic layer as an upper layer 2 are formed on a nonmagnetic substrate 1. In this double- layer coating structure, the thickness of the upper layer is regulated to <=0.5μm and acicular inorg. powder of 0.05-0.2μm major axis size is incorporated into nonmagnetic powder in the lower layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called multilayer coating type magnetic recording medium, and more particularly to improvement of electromagnetic conversion characteristics in a short wavelength region.

[0002]

2. Description of the Related Art As a magnetic recording medium, a magnetic coating is prepared by dispersing a ferromagnetic powder, a binder and various additives together with an organic solvent, and a magnetic layer is formed by coating and drying it on a non-magnetic support. There is known a so-called coating type magnetic recording medium, and metal fine particles have been used as the above-mentioned ferromagnetic powder for the purpose of high density recording.

A coating type magnetic recording medium using such metal fine particles is used as a recording medium for computers such as a magnetic tape for audio or video, a high density floppy disk, a data cartridge for backup, etc. It has become the mainstream of magnetic recording media.

In order to realize high density recording of a coating type magnetic recording medium, fine metal particles are used as a ferromagnetic powder and the medium surface is super smoothed to minimize spacing loss. It is also important to reduce output loss due to recording demagnetization.

As a method for achieving these purposes, (1)
Increase of coercive force and saturation magnetization of ferromagnetic powder, (2) homogenization of coercive force distribution of ferromagnetic powder, (3) imparting perpendicular anisotropy,
(4) The magnetic layer may be thinned.

Of these, (1) and (2) are techniques for directly improving the output. In order to improve the coercive force and the saturation magnetization, the elemental composition of the ferromagnetic powder has been studied, and the metal fine particles having a coercive force of more than 160 kA / m and the saturation magnetization of more than 140 Am ³ / kg. Is also being developed. Also, the coercive force distribution reflects the particle size distribution of the ferromagnetic powder,
By making the particle sizes uniform, the coercive force distribution is also significantly improved.

The imparting of perpendicular anisotropy (3) is a method for increasing the density by perpendicular magnetic recording. In the case of a coating type magnetic recording medium, this is largely due to the control of the magnetic orientation of the ferromagnetic powder. For example, when acicular particles are used, it has been attempted to perform vertical orientation treatment or oblique orientation treatment on the coating film, but it is difficult to control the orientation, and problems such as disturbance of the coating surface due to orientation are caused. However, these treatment methods have not yet become practical.

Next, the thinning of the magnetic layer in (4) is considered to be very effective as a method for reducing self-demagnetization loss. Here, the film thickness of the magnetic layer is, for example, 1 μm.
If the thickness is simply reduced to m or less, the surface shape of the non-magnetic support is likely to appear on the surface of the magnetic layer, making it difficult to smooth the surface of the magnetic layer. For this reason, in the case where the magnetic layer is thinned, a multi-layer coating type configuration in which a non-magnetic coating layer is interposed between the non-magnetic support and the magnetic layer is often adopted. By interposing the non-magnetic layer, the thickness is increased between the surface of the non-magnetic support and the surface of the magnetic layer, and the surface shape of the non-magnetic support hardly appears on the surface of the magnetic layer. Therefore, a thin magnetic layer is formed with a smooth surface shape.

Here, as a method for coating and forming such a non-magnetic layer and a magnetic layer, for example, two slits through which a non-magnetic coating and a magnetic coating are respectively extruded are provided.
A wet multi-layer coating method (wet-on-wet coating method) in which a non-magnetic coating material and a magnetic coating material are simultaneously coated on a non-magnetic support by using a 4-lip type die head is preferable.
According to this wet multi-layer coating method, a coating film having a good surface shape with relatively few coating defects and coating streaks is formed. In addition, the formed lower layer and the upper layer have high adhesion,
Excellent durability is obtained.

[0010]

However, even in the case of a magnetic recording medium having a non-magnetic layer and a magnetic layer formed by a wet multi-layer coating method, if it is used in an existing video device or computer device, its surface property is surely obtained. Is enough. However, in the field of magnetic recording devices, higher density recording is progressing, and it is insufficient to obtain sufficient electromagnetic conversion characteristics on such devices, and further improvement of surface properties is required.

Therefore, the present invention has been proposed in view of such a conventional situation, and it has a good surface property and a magnetic field which can obtain a good electromagnetic conversion characteristic even when a high density recording is achieved. The purpose is to provide a recording medium.

[0012]

The present invention has been completed to achieve the above objects.

That is, the present invention provides a so-called multilayer coating type magnetic recording medium comprising a lower layer in which a non-magnetic powder is dispersed in a binder and an upper layer in which a ferromagnetic powder is dispersed in a binder. Applied.

In the present invention, in such a multi-layer coating type magnetic recording medium, the thickness of the upper layer is regulated to 0.5 μm or less, and the lower layer has a major axis length of 0.
An acicular inorganic powder having a particle size of 05 to 0.2 μm is contained.

When the thickness of the upper layer is 0.5 μm or less, the recording demagnetization is reduced and the output is improved. Further, when the lower layer contains needle-like inorganic powder of 0.05 to 0.2 μm as non-magnetic powder, the surface property of the lower layer is improved, and reflecting this, the surface property of the upper layer formed on the lower layer is also improved. Be improved.
As a result, especially in the short wavelength region, the electromagnetic conversion characteristics such as output and overwrite characteristics are improved. Further, the acicular inorganic powder imparts great strength to the coating film due to the entanglement of the acicular powders, and contributes to the improvement of running durability of the medium.

The acicular inorganic powder further contained in the lower layer preferably has a major axis length of 0.05 to 0.2 μm and an acicular ratio of 2 to 10.

As a result, the surface properties of the lower layer are further improved.

The lower layer has an average particle size of 0.0
You may contain 1-0.04 micrometer carbon black. However, when the lower layer contains carbon black,
The volume ratio of acicular inorganic powder to carbon black is 7
It is desirable to set it to 0:30 to 100: 0.

The use of carbon black imparts appropriate conductivity to the medium and prevents the generation of static electricity when sliding with various sliding members. As a result, running durability is improved.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Specific embodiments of the present invention will be described below.

In the magnetic recording medium of the present invention, a lower layer (nonmagnetic layer) formed by dispersing nonmagnetic powder in a binder and an upper layer formed by dispersing ferromagnetic powder in the binder (on a nonmagnetic support). This is a so-called multilayer coating type magnetic recording medium in which a magnetic layer) is formed.

In this multilayer coating type magnetic recording medium, the lower layer composition and the upper layer composition are respectively dispersed in an organic solvent to prepare a lower layer coating material and an upper layer coating material, and these coating materials are coated on a non-magnetic support and dried. Made in.

In the present invention, in such a multilayer coating type magnetic recording medium, the thickness of the magnetic layer formed as the upper layer is regulated to 0.5 μm or less, and the lower layer has a major axis length of non-magnetic powder. An acicular inorganic powder of 0.05 to 0.2 μm is contained.

The thickness of the magnetic layer formed as the upper layer is set to 0.
When the thickness is regulated to 5 μm or less, recording demagnetization is reduced and output is improved.

However, when the thickness of the magnetic layer is reduced, the surface of the magnetic layer is easily affected by the surface shape of the lower side.
Therefore, if the surface shape on the lower side, that is, the surface shape of the lower layer is rough, the surface property of the magnetic layer is deteriorated and the electromagnetic conversion characteristics are impaired.

Therefore, in the present invention, the thickness of the magnetic layer is set to 0.
The needle-like inorganic powder having a major axis length of 0.05 to 0.2 μm is used as the non-magnetic powder contained in the lower layer while being regulated to 5 μm or less.

Needle-like inorganic powder having a major axis length of less than 0.05 μm is difficult to disperse in a paint, so that when it is contained in a paint film, the paint film surface becomes rough. On the other hand, the acicular inorganic powder having a major axis length of more than 0.2 μm is easily dispersed in the paint. However, in this case, since the powder is present in the coating film in a non-oriented state, the needle-like shape of the needle-like inorganic powder having a long major axis length floats on the coating film surface and deteriorates the surface property. . Thus, the major axis length is 0.05
If an acicular inorganic powder outside the range of .about.0.2 .mu.m is used, the electromagnetic conversion characteristics deteriorate in any case due to spacing loss.

On the other hand, the major axis length is 0.05 to 0.2.
The acicular inorganic powder having a particle size of μm has good dispersibility in a coating material, and the acicular shape does not stand out on the surface of the coating film. Therefore, the surface property of the lower layer is improved by using such acicular magnetic powder having a long axis length, and reflecting this,
The surface property of the magnetic layer formed thereon is also improved. As a result, the electromagnetic conversion characteristics such as output at a short wavelength and overwriting are improved. Further, the acicular inorganic powder imparts great strength to the coating film due to the entanglement of the acicular powders, and contributes to the improvement of running durability.

In order to impart a better surface property to the medium, the acicular inorganic powder has a major axis length of 0.05 to 0.
It is more preferable that the acicular ratio is 2 to 10 and the acicular ratio is 2 to 10. Further, the specific surface area is preferably ^{5 to} 100 m ² / g, and more preferably ²⁰ to 70 m ² / g.

Specific examples of the acicular inorganic powder include non-magnetic iron oxide such as α-Fe ₂ O ₃ , goethite,
Examples of the powder include titanium oxide and zinc oxide. These powders may be used alone or in combination of two or more kinds. In addition, these acicular inorganic powders may be doped with an appropriate amount of impurities, if necessary, to improve dispersibility, impart conductivity, improve color tone, and the like.
The surface treatment may be performed with a compound such as i, Ti, Sn, Sb or Zr.

Further, when carbon black is further added to the lower layer, appropriate conductivity is imparted to the medium, and generation of static electricity is prevented during sliding with various sliding members. However, when carbon black is contained in the lower layer, the volume ratio of the acicular inorganic powder to the carbon black is 70:
It is desirable to set it to 30 to 100: 0. When the volume ratio of carbon black is larger than this range, the smoothness of the lower layer and by extension, the upper layer is impaired. The carbon black has a specific surface area of 100 to 400 m ² / g, D
The BP oil absorption is preferably 20 to 200 ml / 100 g.

Examples of such carbon black include a furnace for rubber, pyrolytic carbon, black for color,
There is acetylene black and the like. These carbon blacks may be used alone or in combination of two or more.

In the present invention, the thickness of the upper magnetic layer and the shape of the needle-like inorganic powder contained in the lower layer are regulated as described above, but the binder and the binder forming the lower layer together with the needle-like inorganic powder and carbon black are used. As the ferromagnetic powder and the binder constituting the upper layer, those usually used in the multi-layer coating type can be used.

First, as the binder used in the lower layer,
Conventionally used are thermoplastic resins, thermosetting resins, reactive resins and the like which have been used as binders for magnetic recording media, and those having a number average molecular weight of 5,000 to 100,000 are particularly preferable.

As the thermoplastic resin, vinyl chloride, vinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, Acrylic ester-vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer,
Acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer, methacrylate-vinyl chloride copolymer, methacrylate-ethylene copolymer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer , Acrylonitrile-
Butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose), styrene butadiene copolymer, polyurethane resin, polyester resin, amino resin, And synthetic rubber.

As the thermosetting resin or reactive resin, phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, polyamine resin, urea formaldehyde resin or the like can be used. is there.

These binders may be used alone or in combination of two or more.

The binder contains --SO ₃ M, --OSO ₃ M and --COO for the purpose of improving the dispersibility of the powder component.
M, P = O (OM) 2 ( where formula, M is a hydrogen atom or a lithium, potassium, an alkali metal such as sodium) _{or, -NR 1 R 2, -NR 1} R 2 R 3 + X - in Side chain type amine represented,> NR ₁ R ₂ ⁺ main chain type amine represented by X ⁻ (wherein R ₁ , R ₂ and R ₃ represent a hydrogen atom or a hydrocarbon group, and X ⁻ is A halogen element ion such as fluorine, chlorine, bromine or iodine, or an inorganic ion or an organic ion), or a polar functional group such as -OH, -SH, -CN or an epoxy group may be introduced. The amount of these polar functional groups introduced into the binder is preferably 10 ^-1 to 10 ^-8 mol / g, more preferably 10 ^-2 to 10 ^-6 mol / g.

The amount of the binder contained in the lower layer is 1 to 200 parts by weight, preferably 10 to 50 parts by weight, based on 100 parts by weight of the non-magnetic powder. If the amount of the binder is too large, the proportion of the non-magnetic powder will decrease, so that the action of the non-magnetic powder will not be sufficiently exerted. On the other hand, if the amount of the binder is too small, the mechanical strength of the coating film will be reduced, and the dispersion of the non-magnetic powder will be poor, and the surface property will be reduced.

Further, these binders may be crosslinked and cured with a polyisocyanate crosslinking agent. Examples of the cross-linking agent include toluene diisocyanate and its adduct, or alkylene diisocyanate and its adduct. The amount of these polyisocyanate crosslinking agents to be compounded is 5 to 80 parts by weight, preferably 10 to 50 parts by weight, based on 100 parts by weight of the binder.

On the other hand, the upper layer is mainly composed of ferromagnetic powder and a binder.

As the ferromagnetic powder, Fe, Co, N
Metals such as i, Fe-Co, Fe-Ni, Fe-Al, F
e-Ni-Al, Fe-Al-P, Fe-Ni-Si-
Al, Fe-Ni-Si-Al-Mn, Fe-Mn-Z
n, Fe-Ni-Zn, Co-Ni, Co-P, Fe-
Co-Ni, Fe-Co-Ni-Cr, Fe-Co-N
i-P, Fe-Co-B, Fe-Co-Cr-B, Mn
Examples include alloys such as —Bi, Mn—Al, and Fe—Co—V, iron nitride, and iron carbide. These ferromagnetic powders contain Al, S, etc. for the purpose of preventing sintering during reduction or maintaining the shape.
Light metal elements such as i, P, and B may be contained in appropriate amounts.

Further, as the ferromagnetic powder, γ-
Fe ₂ O ₃ , Fe ₃ O ₄ , a beltride compound of γ-Fe ₂ O ₃ and Fe ₃ O ₄ , Co-containing γ-Fe ₂ O ₃ , Co-containing Fe
_A beltride compound of γ-Fe ₂ O ₃ and Fe ₃ O ₄ containing ₃ O ₄ , Co and CrO ₂ containing one or more metal elements such as Te, Sb, Fe and B. You may use an oxide etc. Further, hexagonal plate-shaped ferrite can also be used, and M-type, W-type, Y-type, Z-type barium ferrite, strontium ferrite, calcium ferrite, lead ferrite, and these ferrites with Co for the purpose of controlling coercive force. -Ti, Co-Ti-Zn, C
o-Ti-Nb, Co-Ti-Zn-Nb, Cu-Z
It is also possible to use those to which r, Ni-Ti, etc. have been added.

The above ferromagnetic powders may be used alone or in combination of two or more.

The ferromagnetic powder has a specific surface area of 20-9.
0 m ² / g, it is desirable that preferably 25~70m ² / g. Since the ferromagnetic powder having a specific surface area within the above range is appropriately fine, it reduces the noise of the medium and is suitable as a magnetic material for high density recording.

When acicular magnetic powder is used among these ferromagnetic powders, the major axis length is 0.05 to 0.50 μm.
It is desirable that m and the axial ratio be 3 to 15. If the major axis length of the magnetic powder is less than 0.05 μm, it becomes difficult to disperse it in the magnetic paint, and if the major axis length exceeds 0.50 μm, noise may increase. Also, the magnetic powder has an axial ratio of 3
If it is less than the above range, the orientation of the ferromagnetic powder is lowered, leading to a reduction in output. On the contrary, when the axial ratio exceeds 15, the short wavelength signal output may be reduced.

On the other hand, when a plate-shaped ferrite is used as the ferromagnetic powder, it is preferable that the plate diameter is 0.01 to 0.5 μm and the plate thickness is 0.001 μm to 0.2 μm.

The shape parameters of these ferromagnetic powders, that is, the major axis length, axial ratio, plate diameter and plate thickness, were selected randomly from 100 or more samples from transmission electron micrographs.
This average value is adopted.

As the binder forming the upper layer together with these ferromagnetic powders, any of the binders mentioned above as the binder forming the lower layer can be used. The amount of the binder contained in the upper layer is also 1 to 200 parts by weight, preferably 10 to 50 parts by weight, based on 100 parts by weight of the ferromagnetic powder, as in the case of the lower layer. When the amount of the binder is too large, the proportion of the ferromagnetic powder in the upper layer is relatively small, which causes a decrease in output. On the other hand, when the amount of the binder is too small, the mechanical strength of the coating film is lowered and the running durability of the magnetic recording medium is impaired. Furthermore, the dispersibility of the ferromagnetic powder becomes poor, and this also reduces the output.

Even in the case of the upper layer, the binder may be crosslinked and cured with a polyisocyanate crosslinking agent.
The cross-linking agent may be used in both the upper layer and the lower layer, or may be used in only one layer. In addition, a cross-linking agent on the upper layer,
When used in both the lower layer, the amount of the cross-linking agent to be mixed in the upper layer
The lower layers may be the same or different.

As described above, the lower layer and the upper layer are basically composed of the non-magnetic powder and the binder or the ferromagnetic powder and the binder, respectively.
Additives such as non-magnetic reinforcing particles and surfactants may be added.

Examples of the lubricant include solid lubricants such as graphite, molybdenum disulfide, and tungsten disulfide, silicone oil, fatty acids having 10 to 22 carbon atoms, and 10 to 22 carbon atoms.
Fatty acid esters, terpene-based compounds, and oligomers thereof, which are synthesized from the above-mentioned fatty acids and alcohols having 2 to 26 carbon atoms. These lubricants may be added only to the upper layer, or may be added to both the upper layer and the lower layer.

The non-magnetic reinforcing particles include aluminum oxide (α, β, γ), chromium oxide, silicon carbide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide (rutile type). ,
Anatase type) etc. In these non-magnetic reinforcing particles,
The Mohs hardness is 5 or more, preferably 6 or more, the specific gravity is in the range of 2 to 6, preferably 3 to 5, and the average particle diameter is 1.0 μm or less, preferably 0.5 μm or less. Among them, the average particle size of the non-magnetic reinforcing particles is 100 samples or more randomly selected from the transmission electron micrograph,
This average value is adopted. These non-magnetic reinforcing particles are not added to the lower layer, but are added only to the upper layer as needed. An appropriate amount of addition is 20 parts by weight or less, preferably 10 parts by weight or less, relative to 100 parts by weight of the ferromagnetic powder.

As the surfactant, any of nonionic, anionic, cationic and amphoteric surfactants can be used. These surfactants may be added to either the upper layer or the lower layer, or may be added to both the upper layer and the lower layer. When a surfactant is added to both layers, the same type may be used for the upper layer and the lower layer, or different types may be used. Further, the addition amount may be the same or different in the upper layer and the lower layer.

The lower layer and the upper layer as described above are the lower layer composition,
Disperse and knead the upper layer composition in an organic solvent to prepare a lower layer coating material and an upper layer coating material, coat and dry these coating materials on a non-magnetic support, and further subject the upper layer surface to a surface smoothing treatment such as calendering. Made in.

As the solvent for forming the lower layer composition and the upper layer composition into a coating material, a ketone solvent such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, an alcohol solvent such as methanol, ethanol and propanol, methyl acetate and acetic acid. Ethyl, butyl acetate,
Ester solvents such as propyl acetate, ethyl lactate and ethylene glycol acetate, ether solvents such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran and dioxane, aromatic hydrocarbon solvents such as benzene, toluene and xylene, methylene chloride and ethylene. Halogenated hydrocarbon solvents such as chloride, carbon tetrachloride, chloroform and chlorobenzene are used.

Further, as a device for dispersing and kneading the lower layer composition or the upper layer composition with a solvent, a roll mill, a ball mill, a sand mill, an agitator, a kneader, an extruder, a homogenizer, an ultrasonic disperser or the like is used.

The prepared coating material is applied onto a non-magnetic support. As the non-magnetic support, any of those usually used in magnetic recording media can be used, and specifically, polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polyolefins such as polypropylene, and cellulose. There are celluloses such as triacetate and cellulose diacetate, polymer materials typified by vinyl resins, polyimides and polycarbonates, and supports made of metal, glass, ceramics and the like.

Here, as a method for coating the two kinds of prepared coating materials on a non-magnetic support, JP-A-6-2365 is used.
No. 43 publication, a lower layer coating material is applied and dried, and an upper layer coating material is applied on the dried lower layer coating material and dried, which is a so-called wet-on-dry coating method and a wet state. There is a so-called wet-on-wet coating method (wet multi-layer coating method) in which an upper-layer coating is overlaid on a lower-layer coating.

Of these, it is preferable to use the wet-on-wet coating method as the coating method. FIG. 1 shows an example of a coating device that coats a coating material by the wet-on-wet coating method.

This coating apparatus has two slit portions (slit portions 11 for lower layer coating material,
Die head 18 having slit 12) for upper paint
(4 lip type die head). That is, in this die head, the two slit portions 11, 1
A lower paint reservoir 13 and an upper paint reservoir 14 to which a lower paint and an upper paint are supplied, respectively, are formed on the back side of 2.
The lower layer paint and the upper layer paint supplied to the paint pools 13 and 14 are extruded through the slits 11 and 12 to the tip of the die head. On the other hand, the support 15 to which the paint is applied
Travels in the direction A in the figure from the slit 11 for the lower layer paint to the slit 12 for the upper layer paint along the tip surface of the die head.

The non-magnetic support 15 running in this way
First, when passing through the slit portion 11 for the lower layer paint, the lower layer paint extruded from the slit portion 11 is applied to the surface to form the lower layer coating film 16. Then, when passing through the slit portion 12 for the upper layer paint, the upper layer paint extruded from the slit portion 12 is applied on the lower layer coating film 16 in a wet state, and two layers of the coating films 16 and 17 are formed. . Then, the wet two-layer coating film is dried, and if necessary, surface smoothing treatment such as calendering treatment is performed to produce a multilayer coating type magnetic recording medium.

As the die head, there are a 3-lip method, a 2-lip method, etc. in addition to the 4-lip method.

Since the lower layer and the upper layer thus formed by the wet-on-wet coating method are formed by applying the upper layer coating material on the lower layer coating film in a wet state, the surface of the lower layer, that is, The boundary between the lower layer and the upper layer is formed smoothly. Therefore, the surface properties of the upper layer are also very good, dropout is suppressed, high output,
It is suitable for high density recording where low noise is strictly required. Further, since the adhesion between the lower layer and the upper layer is high, film peeling hardly occurs and excellent durability can be obtained.

It should be noted that there is substantially a clear boundary between the lower layer and the upper layer formed by the wet-on-wet coating method, and a boundary in which the components of both layers are mixed with a certain thickness. Areas may exist. In the present invention, when such a boundary region exists, a layer below the boundary region is defined as a lower layer and an upper layer is defined as an upper layer except for the boundary region.

When the upper layer and the lower layer are formed by the wet-on-dry coating method, as a method for applying the lower layer coating material and the upper layer coating material, a die head coating method, a gravure roll coating method, a reverse roll coating method and the like are used. A coating method is adopted. However, in this case, the material of the lower layer needs to be selected so that the lower layer has sufficient solvent resistance to the upper coating material.

The above is the basic structure of the magnetic recording medium of the present invention, but the structure of the magnetic recording medium of the present invention is not limited to this. For example, as with a normal magnetic recording medium, a back coat layer may be provided on the side opposite to the side on which the lower and upper layers of the non-magnetic support are formed for the purpose of improving running properties, preventing electrification and preventing transfer. May be. Further, between the lower layer and the non-magnetic support, it is possible to further provide an undercoat layer on the lower side of the lower layer for the purpose of enhancing the adhesiveness between the lower layer and the non-magnetic support. As the material for the back coat layer and the undercoat layer, any of those usually used in magnetic recording media can be used.

[0068]

EXAMPLES Hereinafter, specific examples of the present invention will be described based on experimental results.

Example 1 The magnetic recording medium produced in this example is shown in FIG. This magnetic recording medium comprises a lower layer 4 and an upper layer 2 on a non-magnetic support 1.
And the back coat layer 3 is formed on the surface opposite to the side on which the lower layer 4 and the upper layer 2 of the non-magnetic support 1 are formed. Such a magnetic recording medium was manufactured as follows.

First, a coating composition was weighed and mixed according to the following upper layer coating composition, kneaded by a kneader, and then dispersed by a sand mill for 4 hours to prepare an upper layer coating composition.

Upper layer coating composition Ferromagnetic powder: Fe-based metal ferromagnetic powder 100 parts by weight (coercive force: 160 kA / m, saturation magnetization: 135 Am ² / kg, specific surface area: 51 m ² / g, major axis length: 0 .15 μm, acicular ratio: 6) Binder: Polyvinyl chloride resin (manufactured by Nippon Zeon, trade name MR-110) 16 parts by weight Polyester polyurethane resin (manufactured by Toyobo Co., Ltd.) 4 parts by weight Additive: carbon 2 parts by weight Al ₂ O ₃ 5 parts by weight Stearic acid 1 part by weight Heptyl stearate 1 part by weight Solvent: methyl ethyl ketone 150 parts by weight Cyclohexanone 150 parts by weight Further, the coating composition was measured and mixed according to the following lower layer coating composition, and similarly. After kneading with a kneader,
The lower layer paint was prepared by dispersing for 4 hours with a sand mill.

Lower layer coating composition Non-magnetic powder: 100 parts by weight Needle-like α-Fe ₂ O ₃ (major axis length: 0.15 μm, needle-like ratio: 6) 89.9 parts by weight Carbon black (average particle size: 0. (025 μm) 10.1 parts by weight (however, the volume ratio (%) of needle-shaped α-Fe ₂ O and carbon black is needle-shaped α-F e ₂ O: carbon black = 70: 30) Binder: Poly Vinyl chloride resin (manufactured by Zeon Corporation, trade name MR-110) 13 parts by weight Polyester polyurethane resin (manufactured by Toyobo Co., Ltd.) 4 parts by weight Additive: stearic acid 1 part by weight Heptyl stearate 1 part by weight Solvent: methyl ethyl ketone 105 parts by weight Cyclohexanone 105 parts by weight Polyisocyanate was added to the upper layer coating material and the lower layer coating material prepared in this way, respectively, and 4 parts by weight of the upper layer coating material and 2 parts of the lower layer coating material were added. Added in parts. Then, the upper layer coating material and the lower layer coating material were applied on a PET (polyethylene terephthalate) film having a thickness of 7 μm using a 4-lip type die coater, and then an orientation treatment was performed by a solenoid coil. After that, the coating film was dried and then subjected to a calendar treatment and a curing treatment in this order. The film thickness is 0.3 μm for the upper layer and 2.4 μm for the lower layer.
It is.

Next, a back coating material was prepared according to the following composition.

Back coating composition Carbon black (Asahi Co., # 50) 100 parts by weight Polyester polyurethane 100 parts by weight (Nipporan product name N-2304) Solvent: Methyl ethyl ketone 500 parts by weight Toluene 500 parts by weight A back coat layer was formed by applying the coating material on the surface opposite to the side where the lower layer and the upper layer of the PET film were formed and drying.

A magnetic tape was produced by slitting the raw tape on which the lower layer, the upper layer and the back coat layer were formed in this way into a width of 8 mm.

Examples 2 and 3 In the lower layer paint, α-F having the shape shown in Table 1 as non-magnetic powder was used.
except that it contained e ₂ O ₃ is A magnetic tape was produced in the same manner as in Example 1.

Comparative Example 1 and Comparative Example 2 Same as Example 1 except that the lower layer paint contained α-Fe ₂ O ₃ and carbon black different from the predetermined shape as non-magnetic powder as shown in Table 1. Then, a magnetic tape was produced.

Comparative Example 3 A lower layer coating material was applied onto a non-magnetic support by a 2-lip type die head having one slit portion, dried, and then the upper layer coating material was applied onto the lower layer using the same die head. A magnetic tape was produced in the same manner as in Example 1 except that the coating (wet-on-dry coating method) was performed.

With respect to the magnetic tape manufactured as described above, the surface roughness Ra of the upper layer surface was measured and
The playback output in Hz was measured.

The surface roughness Ra is measured according to JIS B 06.
The center line average roughness Ra is defined by 01 and is measured here by an optical method.

The reproduction output was measured using a fixed head type electromagnetic conversion characteristic measuring instrument. This fixed head type electromagnetic conversion characteristic measuring instrument has a rotating drum and a head in contact with the rotating drum, and the magnetic tape whose characteristics are measured is wound around the rotating drum and runs along the magnetic tape to slide on the head. Be moved. In this example, a square wave signal of 7 MHz was recorded on each magnetic tape at an optimum recording current using this measuring device, and the output level of this signal was measured by a spectrum analyzer. The output level is shown as a relative value when the output level of the reference tape (8 mm Hi8 tape manufactured by Sony Corporation) is 0 dB.

The surface roughness Ra thus measured and the reproduction output at 7 MHz are shown in Table 1 together with the forming conditions of the upper layer and the lower layer.

[0083]

[Table 1]

Comparing the magnetic tapes of Examples 1 to 3 with the magnetic tapes of Comparative Examples 1 and 2, good surface properties were obtained with the magnetic tapes of Examples 1 to 3,
The reproduction output is also a high value of 3 dB or more. On the other hand, the magnetic tapes of Comparative Examples 1 and 2 in which the major axis length and the needle ratio of the needle-shaped α-Fe ₂ O ₃ contained in the lower layer are outside the predetermined range have a rough surface and a low reproduction output. It is a value. In particular, the magnetic tape of Comparative Example 2 having a large average particle diameter of carbon black of 0.1 μm has such a tendency.

From this, the major axis length of the needle-shaped inorganic powder contained in the lower layer is 0.05 to 0.2 μm, and the needle-shaped ratio is 2 to
It has been found that the regulation of 10 is effective in improving the surface property of the medium and improving the electromagnetic conversion characteristics.

In Comparative Example 3, the upper layer and the lower layer were formed by the wet-on-dry method, but in this case, there were many coating defects and the surface roughness Ra was too high to be measured. Therefore, the spacing loss is large, and the reproduction output cannot be measured. From this, it was found that it is difficult to form the upper layer with a thin film thickness of about 0.3 μm by the wet-on-dry coating method.

The average particle size of carbon black and the addition amount
As a non-magnetic powder, the major axis length is 0.15 μm and the acicular ratio is 8
Using carbon black having an average particle diameter shown in the alpha-Fe ₂ O ₃ and Table 2, to prepare a magnetic tape except for adding to the paint in the same manner as in Example 1 with the addition amount shown them in the table ( Experimental Examples 1 to 6).

Then, the surface roughness Ra of the upper surface of the produced magnetic tape was measured, and the reproduction output at 7 MHz was measured. The surface roughness Ra thus measured and the reproduction output at 7 MHz are shown in Table 2 together with the formation conditions of the upper layer and the lower layer.

[0089]

[Table 2]

In Table 2, first, when the magnetic tapes of Experimental Examples 1 to 4 in which the amount of carbon black added was changed, the magnetic tapes of Experimental Examples 2 to 4 in which the volume ratio of carbon black was 30% or less were compared. In the magnetic tape, a high reproduction output of 3 dB or more is obtained, whereas in the magnetic tape of Experimental Example 1 in which the volume ratio of carbon black is as large as 40%, the reproduction output is as low as 1.8 dB.

Further, comparing Experimental Example 5 and Experimental Example 6 in which the average particle diameter of carbon black was changed with respect to Experimental Example 3, Experimental Example 3 and Experiment in which the average particle diameter of carbon black was 0.04 μm or less In the magnetic tape of Example 5, a sufficient reproduction output of about 3 dB can be obtained, whereas in the magnetic tape of Experimental Example 6 in which the carbon black has a large average particle size of 0.1 μm, a reproduction output as low as 1 dB can be obtained. .

From this, it can be seen that when carbon black is used, it is desirable to optimize its average particle size and content. That is, carbon black has an average particle diameter of 0.01 to 0.04 μm, and the volume ratio of acicular α-Fe ₂ O ₃ to the carbon black is 70:30 to 10.
Suitably 0: 0.

[0093]

As is apparent from the above description, in the present invention, in the multilayer coating type magnetic recording medium, the thickness of the magnetic layer formed as the upper layer is regulated to 0.5 μm or less, and The major axis length of non-magnetic powder is 0.05 ~
Since the acicular inorganic powder of 0.2 μm is used, a thin magnetic layer can be formed with good surface properties. Therefore, good electromagnetic conversion characteristics can be obtained even when high density recording is achieved.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a coating apparatus for forming a lower layer and an upper layer.

FIG. 2 is a schematic cross-sectional view of an essential part showing one structural example of a magnetic recording medium to which the present invention is applied.

[Explanation of symbols]

 1 non-magnetic support 2 upper layer 4 lower layer

Front page continuation (72) Inventor Haruaki Ishizaki 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation

Claims (5)

[Claims] 1. A magnetic recording medium comprising a non-magnetic support and a lower layer having a non-magnetic powder dispersed in a binder and an upper layer having a ferromagnetic powder dispersed in the binder. Has a thickness of 0.5 μm or less, and in the lower layer,
A magnetic recording medium characterized by containing acicular inorganic powder having a major axis length of 0.05 to 0.2 μm as the non-magnetic powder. 2. The magnetic recording medium according to claim 1, wherein the acicular inorganic powder has an acicular ratio of 2 to 10. 3. The magnetic recording medium according to claim 1, wherein the lower layer contains carbon black having an average particle diameter of 0.01 to 0.04 μm as nonmagnetic powder. 4. The magnetic recording medium according to claim 3, wherein the volume ratio of the acicular inorganic powder and carbon black in the lower layer is 70:30 to 100: 0. 5. The magnetic recording medium according to claim 1, wherein the lower layer and the upper layer are formed by a wet multi-layer coating method.